An hour a day during lockdown would be ok (responding to PP), particularly if no other work set by school (as implied in PPs message).
HOWEVER, back in normal times, when school is on (i.e. now, again), I found an hour a week was more than enough! - break it down into 10 minute tests if you wish.
Ramp up the time if they are going very slowly. But it's not always about time spent, but rather, what goes in, and what goes on, and whether they grasp things - or not! (Important lesson learnt by me!)
Also, no need for an assessment: I went for one of these, and the guy charged £70, and told me what I already knew... bottom line: if your child is deemed exceeding in all three areas (or at least two of the areas) in his school report, then go for the academic schools. Also, if your child actually enjoys say maths or reading, then again, go for it. But be aware that if they do not like academic work then, even if they pass, they may feel out of their depth at an academic secondary school (a horrible thing!). Plus, worth remembering that state comprehensives top sets are equivalent to most grammars, plus comps are looked very kindly upon by universities (and suspect this will be more of a trend in future years as society tries to become more equal).
Finally, don't get caught up in the anxiety hype. Websites, tutors, tutor-groups, other parents, they all try to install worry into us parents. With the exception of other parents, they want your money! A bright kid who is exposed to the syllabus will be OK - this is not rocket science, this is the 11+ - and your child CAN work alone, on a work sheet or book, during that hour a week.
Bond books are a good place to start, and then mix it up with other books and even some past papers (school websites tend to reproduce samples) as the year progresses.